I know the market in Brussels a bit, but what do you mean by part-time? You mean short-term? Like three to six months at a time? If so, you might find something Jan - June. If by part time you mean few hours/weekly, you could probably find that by cobbling together some private language school work plus private students, but it will definitely be hard to make ends meet.

Have you got accomodations in these three cities? If you are considering something like living with friends/relatives then it could be do-able.

Your online cert is quite a disadvantage in the entire European region....most everyone on this job market has CELTA or equivalent.

Part-time as in lowish hours. I'm looking to volunteer or intern in one of those three cities so I can network my way in to a job in the field I'm interested in working in but it'd be nice to have some cash coming in. I have enough cash saved to pay for rent for a year+.

Experience in Korea probably won't help you much - most employers in the European region consider that experience in Asia doesn't relate very closely to that in Europe. The classroom approaches and methods are somewhat different, as is the role of students and their expectations.

You might really consider beefing up your CV with a CELTA - I think you will be at some disadvantage with only the online cert and Korea experience in comparison to most teachers on the job market in this region. It's a bit of an investment, but not too extreme - should pay off over the period of a year or two. Cheapest way would likely be to find a CELTA in the city where you decide to live/work, or to choose some provider in a really cheap country before you get back to Europe, if you're not here already.

I'm sure there is part time work available in all the cities you mention-I'd pick the one with the best current economy, though - and that might not be Brussels. It's likely the 'poorest' of the three cities you mention in terms of wages.